Income from Continuing Operations Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Basic income from continuing operations per common share:
Income from continuing operations	$ 775	$ 769	$ 810
Less: Income from continuing operations allocable to participating securities	(9)	(8)	(8)
Income from continuing operations allocable to common shares	$ 766	$ 761	$ 802
Weighted average common shares outstanding	414	426	439
Basic income from continuing operations per common share (in dollars per share)	$ 1.85	$ 1.79	$ 1.83
Diluted income from continuing operations per common share:
Income from continuing operations	$ 775	$ 769	$ 810
Less: Income from continuing operations allocable to participating securities	(9)	(8)	(8)
Income from continuing operations allocable to common shares	$ 766	$ 761	$ 802
Weighted average shares outstanding and common share equivalents:
Weighted average common shares outstanding	414	426	439
Weighted average effect of share-based payment awards	1	1	2
Denominator in calculation of diluted income per share	415	427	441
Diluted income from continuing operations per common share (in dollars per share)	$ 1.85	$ 1.78	$ 1.82
Income from continuing operations per common share, Other disclosures [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from calculation	1	2	1
Weighted average restricted stock awards considered participating securities	5	4	4